I measured mine and it appears to be a 3/8" ID x 7/16" OD but it might be a metric size close to those dimensions. You should be able to get that from any machine shop that does pneumatic or hydraulic fittings. Any O-ring intended for pneumatic or hydraulic fittings will be compatible with Super Lube.
EDIT: It seems that 3/8" X 7/16" is not a standard size, so it's probably a metric 9.50 ID x 0.80 W